3phase lighting and shared neutral

Not exactly DIY guys but something ive come across and would appreciatte some comments from anyone who has experience of such circuits. Been reading posts on the net and other forums regarding the subjetct.


The situation i have encountered (which im on with doing some planning for) involves a 3phase lighting circuit wired as follows,

A 32A TP mcb supplying a submain mini CU. The mini CU houses 3 x single pole 20A mcb's each supplied by a phase from the 32A breaker. A 4 core swa cable runs from this mini CU up into the roof to supply light fittings. Each phase of the swa is connected to one of the 20A mcbs and each phase supplies 6 400W SON fittings. The other core of the swa is used as the neutral which is a shared neutral between the 3 phases.

As far as i understand from 7671 this circuit complies (dont have my copy on me so i cant refer any regs at minute)

Now several points seem to be raised in previous internet posts about the compliance of this setup. These are as follows

1, The OPD protecting the 'circuit' needs to be linked i.e a TP MCB.In this instance the circuit has a TP isolation device,but it has further downstream SP devices (I.E the 20A devices). Anyone see a problem?

2, Previous posts on this topic seem to mistake this shared neutral as a borrowed neutral which are two different things imo.

3, The balancing of the phases. With all SP devices switched on then the loads should be balanced so fine. Even with one switch turned off the Neutral conductor is still adequetlly sized to carry the imbalance as far as i can see it.

4, Loss of neutral on circuit. Whats the worst effect this could have on the light fittings. Nothing too detremental as far as i can see.

Apologies for the long post but its not a circuit setup im used to and am keen to educate myself about it.

Regards.

if 1 breaker trips it immediately inballances the load and the neutral current starts to climb... ( to what exactly I can't be bothered to work out.. need phasor diagrams etc.. )
same thing happens when lamps start to fail.. a bigger load on 2 of the phases than the other one..

switching on of the lamps.. ? where are the switches and can you guarantee that the user will switch all 3 sets of lights on?

The 3 single MCBs should be linked, otherwise if one or two are off, the live conductors will still be connected to live via the neutral.
Easily fixed by replacing the 3 single MCBs with a 3 pole one. Or might be possible to remove them altogether and use the upstream 32A MCB, depending on cable sizes.

Sharing the neutral for all 3 phases is not a problem, even if the loads are not balanced.

Loss of neutral could cause voltages of up to 400V to be applied across various parts of the circuits, depending on where the fault was and how well balanced the phases were. Not good and might cause damage. However this could happen in any 3ph installation which includes single phase loads, so nothing unusual in this setup.

industryspark said:

4, Loss of neutral on circuit. Whats the worst effect this could have on the light fittings. Nothing too detremental as far as i can see.

Click to expand...

If you was to lose the supply neutral, Would you not get 400 volt across the fittings and cause serious damage to the control gear.

It either needs to have a linked TP MCB, or a seperate neutral to each of the single phase circuits.

As it stands at the moment, it is not hugely dangerous, but it is not really right either.

This setup is prohibited by the second part of regulation 314.4

If you were to swap the three SP MCBs for a TP flavored one, you also then risk none compliance with reg 314.1 (iii)

The failure of the neutral is not too much of an issue. It might blow a few fittings up, but it's no different to a single neutral conductor on a proper TP&N submain.

These are just my opinions on this setup, and I'd wait to see what the consensus is first.

333rocky333 said:

industryspark said:

4, Loss of neutral on circuit. Whats the worst effect this could have on the light fittings. Nothing too detremental as far as i can see.

Click to expand...

If you was to lose the supply neutral, Would you not get 400 volt across the fittings and cause serious damage to the control gear.

Click to expand...

not in a ballanced load..
they would still have all their neutrals linked together.. think star point of a motor..

flameport said:

The 3 single MCBs should be linked, otherwise if one or two are off, the live conductors will still be connected to live via the neutral.

Click to expand...

by that logic.. so are the lights upstairs in your house when you turn the downstair breaker off.. the neutrals are joined together at the CU..

how local is the TP breaker upstream of the mini CU? and does the mini CU have a TP isolator?

RF Lighting wrote

It either needs to have a linked TP MCB, or a seperate neutral to each of the single phase circuits.

Click to expand...

Not got my book on me but 7671 does state that the setup i describe is compliant, or it does to how i interpretted it. I did only read the reg once however so might stand to be corrected. I'll find the reg out tomoorow.

There was another reg (559........something) stating something along the lines of a 3 phase lighting circuit requiring ' a device that is capable of isolating all 3 phases'

I cant remember the exact wording but could the 32A TP mcb not be the device? And whats to say the device mite not be a simple isolator (and not an mcb)?

Is this circuit any different from a 3 phase neutral circuit supplied from a TPN fuseboard where one fuse could be removed and two left in?

Ill have a better look through the bible tomorrow but was just canvassing opinion really.

I dont see a lost Neutral as a problem really as others have stated, its no different from a lost neutral elsewhere.

Regards

Coljack wrote:

how local is the TP breaker upstream of the mini CU? and does the mini CU have a TP isolator?

Click to expand...

The TP breaker is in a board in a different workshop to the mini CU, yet only about 15 metres away.

The 'mini cu' as i have called it doesnt have a TP isolator no. Just an enclosure with 3x single pole 20A mcb's in (they are also been used as the functonial switches).

The circuit has other issues that i have solutions for but im very limited as to what i can do with regards to cabling hence me querying this setup.

Regards.

ColJack said:

by that logic.. so are the lights upstairs in your house when you turn the downstair breaker off.. the neutrals are joined together at the CU..

Click to expand...

If the neutrals remain connected then there would be no voltage on the neutral.
The danger on the 3ph circuits could occur if someone diconnects the neutral at one of the light fittings after isolating at only one of the MCBs.

then I'd say it didn't comply with

559.6.2.3 Groups of luminaires divided between the three line conductors of a three-phase system with only one common neutral conductor shall be provided with at least one device that simultaneously disconnects all line conductors.

Click to expand...

..
although it has a means upstream of the CU, you could likewise argue that the large isolator in the local substation makes it comply as that disconnects all the line conductors...
if it's not in the CU that houses the breakers, or is indeed the breakers itself, then it should be downstream..

Flameport wrote:

The danger on the 3ph circuits could occur if someone diconnects the neutral at one of the light fittings after isolating at only one of the MCBs.

Click to expand...

But isnt this true of other TPN circuits which we deem compliant?

I knew this one would cause some debate . Its a funny one because in some ways its no different to other 3 phase neutral circuits imo, yet i do agree that it would be better off as 3 phases and 3 neutrals, i.e seperate circuits. Not an option to rip it out and start again though.

Regards.

Personally I'd label the box with a warning that the neutral was shared and that all breakers should be off before working on the lights.

flameport said:

ColJack said:

by that logic.. so are the lights upstairs in your house when you turn the downstair breaker off.. the neutrals are joined together at the CU..

Click to expand...

If the neutrals remain connected then there would be no voltage on the neutral.
The danger on the 3ph circuits could occur if someone diconnects the neutral at one of the light fittings after isolating at only one of the MCBs.

Click to expand...

yes but if someone disconnected the neutral at the CU and switched one breaker off then yessyou would in effect have 2 ( sets of ) lights wired in series across 2 phases, giving you 400V across 2 lights in series, which is still only 200V across each ( set of ) light..